Males have enormously reduced fertility.

In your world, the chance of any given man fathering a child on any given attempt is next to nil. Your men are nearly infertile. This is not genetic because the women are quite fertile; it is male specific. Perhaps they produce almost no sperm.

The result is that many, many sexual encounters must take place before conception very eventually occurs. Getting these encounters to take place is also problematic for the woman - the men are not very energetic in this regard, possibly as a result of the same problem which reduces sperm count. For a woman to become pregnant, she must try multiple times and given the lack of enthusiasm men in your world have for the endeavor, multiple husbands improve her odds of getting pregnant or at least trying to get pregnant.

The other result of this system is that all the husbands will consider any child to have been fathered by their cooperative efforts (such as they are) and so each has an equal stake in the child. This means more parental care and resources for the child.

Males have enormously reduced fertility.

In your world, the chance of any given man fathering a child on any given attempt is next to nil. Your men are nearly infertile. This is not genetic because the women are quite fertile; it is male specific. They produce almost no sperm. The reason for this is not known but may be due to their consumption of vast quantities of cannabis.

The result is that many, many sexual encounters must take place before conception very eventually occurs. Getting these encounters to take place is also problematic for the woman - the men are not very energetic in this regard, possibly as a result of the same problem which reduces sperm count. For a woman to become pregnant, she must try multiple times. Given the lack of enthusiasm men in your world have for the endeavor, multiple husbands improve her odds of getting pregnant or at least occasionally trying to get pregnant.

The other result of this system is that all the husbands will consider any child to have been fathered by their cooperative efforts (such as they are) and so each of these many placid and gentle daddies has an equal stake in the child. This means more parental care and resources for the child.